The subject invention relates to threading apparatus for sewing machines and in particular an adjustable thread tension device which includes, in general, a plurality of thread friction washers or discs between which the thread is placed during operation of the sewing machine. The thread friction discs may be spring biased towards one another with the spring tension being adjustable so that the tension on the thread may be varied. A mechanism of this type is disclosed in U.S. Pat. No. 3,667,414, issued on June 6, 1972, and assigned to the same assignee as the subject application. Tension devices of the type referred to herein include a tension transmission mechanism through which an operator by adjusting a dial or the like may vary the tension on the thread. The operator adjustment is transmitted through a lever mechanism and to an axially slidable member which is operably connected to the thread tension discs through a spring means for varying the relative position of the discs with respect to one another as well as for varying the spring tension exerted by spring means associated with the tension discs. The devices of this type which preceded the present invention are relatively complex in nature using several coil type springs and adjustment means therefor which makes them costly and difficult to assemble. Furthermore, prior tension devices of this type provide only a linear relationship between dial setting and resulting thread tension.
It is a purpose of the present invention to provide a thread tensioning apparatus of the type generally referred to which is simpler in construction, easier to assemble, relatively less costly, and in which predetermined nonlinear relationship between dial setting and resulting thread tension is readily attainable.